1. Field of the Invention
The present invention is directed to an image signal compressing device for compressing a color still image in accordance with a JPEG (Joint Photographic Expert Group) algorithm.
2. Description of the Related Art
A standardized algorithm for the encoding of high resolution images and the transfer of that information through telecommunication transmission channels has been recommended by the JPEG. In order to enable a large-scale data compression, the baseline process of the JPEG algorithm breaks down the original image data into components on a spatial frequency axis using two-dimensional DCT transformation. Thereafter, data expressed on the spatial frequency axis is quantized, and the quantized data is encoded. In the JPEG encoding algorithm, a predetermined Huffman table is recommended.
In a conventional image signal compressing device, a default quantization table is usually used. Depending upon the object to be photographed, a modified quantization table is obtained by multiplying each quantization coefficient included in the quantization table by a single coefficient.
Since the modified quantization table is obtained by multiplying the single coefficient uniformly in each of the spatial frequencies, a quantization which is in conformity with the characteristics of the image (e.g., when there are many high frequency components in comparison with a low frequency component) cannot be performed. Thus, the image data cannot be compressed without reducing the image quality.